Current driven polyphase filters and method of operation

ABSTRACT

A filter circuit includes a polyphase filter coupled to a first circuit component and a second circuit component. The first circuit component propagates four input current signals to the polyphase filter. The third and fourth input current signals are substantially one-hundred-eighty degrees out of phase with the first and second input current signals. The polyphase filter receives the input current signals and generates four output current signals that are out of phase with each other. The second circuit component generates first and second output signals using the four output current signals. The first and second output signals are ninety degrees out of phase with each other.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/232,351 entitled, “Current Driven Polyphase Filters and Method ofOperation,” Attorney's Docket No. 073671.0109, filed Aug. 30, 2002.

TECHNICAL FIELD OF THE INVENTION

This invention relates to circuits and more particularly to currentdriven polyphase filters.

BACKGROUND OF THE INVENTION

Polyphase filters are commonly used in communication systems forgenerating quadrature local oscillator (LO) signals from a single LOsignal. Prior polyphase filters are driven by a voltage buffer and arecharacterized by a low impedance input. A drawback to this approach isthat the voltage buffers require a significant amount of power in orderto drive the polyphase filter linearly. Otherwise, third orderdistortion may result.

SUMMARY OF THE INVENTION

In accordance with the present invention, the disadvantages and problemsassociated with prior polyphase filters have been substantially reducedor eliminated.

In accordance with one embodiment of the present invention, a filtercircuit includes a polyphase filter coupled to a first circuit componentand a second circuit component. The first circuit component propagates afirst input current signal, a second input current signal, a third inputcurrent signal, and a fourth input current signal. The third and fourthinput current signals are substantially one-hundred-eighty degrees outof phase with the first and second input current signals.

The polyphase filter receives the first, second, third, and fourth inputcurrent signals. The polyphase filter further generates a first outputcurrent signal; a second output current signal that is substantiallyninety degrees out of phase with the first output current signal; athird output current signal that is substantially one-hundred-eightydegrees out of phase with the first output current signal; and a fourthoutput current signal that is substantially two-hundred-seventy degreesout of phase with the first output current signal.

The second circuit component generates a first output signal using thefirst output current signal and the third output current signal. Thefirst output signal is at a first phase. The second circuit componentfurther generates a second output signal using the second output currentsignal and the fourth output current signal. The second output signal isat a second phase that is substantially ninety degrees out of phase withthe first phase.

Another embodiment of the present invention is an image frequencyrejection circuit that includes a first polyphase filter, a first mixer,a second mixer, and a second polyphase filter communicatively coupled tothe first mixer and the second mixer. The first polyphase filtergenerates a first LO signal and a second LO signal that is substantiallyninety degrees out of phase with the first LO signal. The first mixerreceives an RF signal and the first LO signal and communicates a firstIF current signal, a second IF current signal that is substantiallyone-hundred-eighty degrees out of phase with the first IF currentsignal, and at least a portion of an image frequency signal. The secondmixer receives the RF signal and the second LO signal and communicates athird IF current signal that is substantially ninety degrees out ofphase with the first IF current signal, a fourth IF current signal thatis substantially two-hundred-seventy degrees out of phase with the firstIF current signal, and at least a portion of the image frequency signal.

A phase shift associated with the first polyphase filter and a phaseshift associated with the second polyphase filter combine to cancel theimage frequency signal such that the second polyphase filtercommunicates a first IF output current signal, a second IF outputcurrent signal, a third IF output current signal, and a fourth IF outputcurrent signal. The second IF output current signal is substantiallyone-hundred-eighty degrees out of phase with the first IF output currentsignal. The third IF output current signal is substantially in phasewith the first IF output current signal. The fourth IF output currentsignal is substantially one-hundred-eighty degrees out of phase with thefirst IF output current signal.

The following technical advantages may be achieved by some, none, or allof the embodiments of the present invention. Technical advantages of thecurrent driven polyphase filter include a reduction in distortion andpower savings. In particular, the current driven polyphase filter isdriven linearly, such as by a voltage-to-current converter circuit,without significant power consumption. In this regard, the currentdriven polyphase filter generally does not receive and operate uponsignals at the third order harmonic, or any other harmonic frequency, ofthe fundamental frequency. As a result, third order distortion isreduced, if not eliminated. These and other advantages, features, andobjects of the present invention will be more readily understood in viewof the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsadvantages, reference is now made to the following description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 illustrates one embodiment of a current driven polyphase filter;and

FIG. 2 illustrates one embodiment of an image frequency rejectioncircuit using the current driven polyphase filter of FIG. 1.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates one embodiment of a filter circuit 10 that includes afirst converter circuit 12, a current driven polyphase filter 14, and asecond converter circuit 16. In general, circuit 10 receives adifferential input voltage signal comprising first input voltage signal20 and second input voltage signal 22, and generates a firstdifferential output voltage signal comprising output voltage signals 24and 26 at a first phase (e.g. 0 degrees) and a second differentialoutput voltage signal comprising voltage signals 28 and 30 at a secondphase (e.g., 90 degrees) that is phase shifted by ninety degrees fromthe first phase. In this regard, circuit 10 receives an input andgenerates a quadrature output.

Although the following description of circuit 10 is detailed withrespect to differential input signals and differential output signals,it should be understood that circuit 10 and the current driven polyphasefilter 14 of circuit 10 may operate upon single-ended input signals togenerate single-ended quadrature output signals without departing fromthe scope of the present invention.

First converter circuit 12 comprises a voltage-to-current convertercircuit that includes a first transistor 40, a second transistor 42, athird transistor 44, and a fourth transistor 46. Although thedescription of transistors 40-46 is detailed with respect to (npn)Bipolar Junction Transistors (BJTs), it should be understood thattransistors 40-46 may comprise any suitable combination of (pnp) BJTs,Field Effect Transistors (FETs), Metal-Oxide Semiconductor Field EffectTransistors (MOSFETs) or any other suitable transistor. The base offirst transistor 40 and the base of second transistor 42 are eachcoupled to first input voltage signal 20. The base of third transistor44 and the base of fourth transistor 46 are each coupled to second inputvoltage signal 22. First and second input voltage signals 20 and 22comprise substantially the same amplitude over a substantially similarrange of frequencies, but are one-hundred eighty (180) degrees out ofphase with each other. In a particular embodiment, signals 20 and 22 aregenerated by local oscillators at local oscillator frequencies.

In one embodiment, a resistor 50 couples the emitter of first transistor40 with the emitter of third transistor 44. A resistor 52 couples theemitter of second transistor 42 with the emitter of fourth transistor46. Resistors 50 and 52 generally help circuit 12 operate linearly.Current sources 54-60 are coupled to the emitters of transistors 40-46,respectively. Circuit 12 receives input voltages 20 and 22 and generatesfour input current signals 62, 64, 66, and 68, (e.g., in-phase signal(I); quadrature signal (Q); in-phase complement signal (-I); andquadrature complement signal (-Q)). Signals 66 (-I) and 68 (-Q) are onehundred eighty degrees out of phase with signals 62 (I) and 64 (Q).Current signals 62-68 are communicated to polyphase filter 14.

In some embodiments, the amplitude of input current signals 64 (Q) and68 (-Q) are zero. This variation in amplitudes is achieved, in oneembodiment, by cutting the metal trace 120 between the emitters oftransistors 40 and 42, indicated by dashed lines, and by cutting themetal trace 122 between the emitters of transistors 44 and 46, indicatedby dashed lines.

Polyphase filter 14 comprises a network of resistors and capacitorsarranged in stages 80 to form a complex bandpass filter that receivesinput current signals 62, 64, 66, and 68, and generates output currentsignals 90, 92, 94, and 96 (e.g., I, Q, -I, and -Q) which are ninetydegrees phase shifted apart. For example, signal 92 (Q) is ninetydegrees out of phase with signal 90 (I). Signal 94 (-I) is one-hundredeighty degrees out of phase with signal 90 (I). Signal 96 (-Q) istwo-hundred-seventy degrees out of phase with signal 90 (I). In general,polyphase filter 14 is tuned to operate over a range of frequencies suchthat it receives input signals at a fundamental frequency within therange of frequencies and generates the phase shifted output signals90-96 at the fundamental frequency.

In a particular embodiment, polyphase filter 14 comprises three stages80 a, 80 b, and 80 c of resistors and capacitors. Stages 80 a-c aregenerally referred to as stages 80. Although polyphase filter 14 isillustrated with three stages 80 of resistors and capacitors, it shouldbe understood that polyphase filter 14 may comprise any suitable numberof stages 80 to perform its operation. In general, the resistor andcapacitor values within any given stage 80 are substantially the same.However, in one embodiment, first stage 80 a has a higher impedance thansecond stage 80 b, and second stage 80 b has a higher impedance thanthird stage 80 c.

In one embodiment, second converter circuit 16 comprises acurrent-to-voltage converter circuit that includes a first transistor100, a second transistor 102, a third transistor 104, and a fourthtransistor 106. Although the description of transistors 100-106 isdetailed with respect to (npn) Bipolar Junction Transistors (BJTs), itshould be understood that transistors 100-106 may comprise any suitablecombination of (pnp) BJTs, Field Effect Transistors (FETs), Metal-OxideSemiconductor Field Effect Transistors (MOSFETs) or any other suitabletransistor. The base of each transistor 100-106 is coupled to a biasvoltage 108. The emitter of each transistor 100-106 is coupled topolyphase filter 14. The collector of each transistor 100-106 is coupledto a corresponding resistor 110-116 and Vcc. Output voltage signals 24,28, 26, and 30 are tapped at the node between the collector of eachcorresponding transistor 100-106 and the corresponding resistor 110-116.Output voltage signals 24 (I) and 26 (-I) form a first differentialvoltage signal at a first phase (e.g., 0 degrees). Output voltagesignals 28 (Q) and 30 (-Q) form a second differential voltage signal ata second phase (e.g., 90 degrees) that is phase shifted by ninetydegrees from the first phase.

Although the description of second converter circuit 16 is detailed withrespect to a current-to-voltage converter circuit, it should beunderstood that converter circuit 16 comprises any suitable circuitrythat converts current to voltage, current to power, or current toamplified current. For example, circuit 16 may comprise a transimpedanceamplifier, a current amplifier, or any other suitable device tomanipulate the form of signals 90-96.

In prior voltage driven polyphase filters, the current-to-voltageconverter circuit 16 is combined with the voltage-to-current convertercircuit 12 in a voltage buffer that drives the polyphase filter. It istypically the current-to-voltage converter circuit 16 that contributessignificantly to the creation of input signals at harmonic frequencies,such as the third order harmonic, of the fundamental frequency. As aresult, a voltage driven polyphase filter operates upon input signals atharmonic frequencies, resulting in distortion in the output signals ofthe polyphase filter. Therefore, in prior voltage driven polyphasefilters where the current-to-voltage converter circuit 16 was arrangedat the input of the polyphase filter, the voltage buffer requiredsignificant power in order to drive the polyphase filter linearly.

A particular advantage of polyphase filter 14 of circuit 10 is that itreceives and operates upon input signals 62-68 in the current domain. Incircuit 10, current-to-voltage converter circuit 16—the primary sourceof signals at harmonic frequencies—is arranged at the output ofpolyphase filter 14. Therefore, current driven polyphase filter 14 doesnot receive and operate upon signals at the third order harmonic, or anyother harmonic frequency, of the fundamental frequency. As a result, thedeleterious effects of the current-to-voltage circuit 16 have no impacton the phase shifting operations of current driven polyphase filter 14in circuit 10. In this regard, third order distortion is reduced, if noteliminated. Moreover, current-to-voltage converter circuit 16 arrangedat the output of polyphase filter 14 requires less power to drivesubsequent circuit components than it does when it is arranged to drivea voltage driven polyphase filter. In this regard, circuit 10 realizespower savings.

Although polyphase filter 14 is described in FIG. 1 with reference to aLocal Oscillator (LO) polyphase filter, it should be understood thatcurrent driven polyphase filter 14 may also comprise an IntermediateFrequency (IF) polyphase filter. IF polyphase filter 14 also receivesinput current signals 62 (I), 64 (Q), 66 (-I) and 68 (-Q) and generatesoutput current signals 90 (I), 92 (Q), 94 (-I), and 96 (-Q). However,the phase relationships of input current signals 62-68 and outputcurrent signals 90-96 associated with IF polyphase filter 14 aredifferent from those associated with LO polyphase filter 14.

For example, with respect to input current signals 62-68 associated withIF polyphase filter 14, signal 64 (Q) is ninety degrees out of phasewith signal 62 (I). Signal 66 (-I) is one-hundred eighty degrees out ofphase with signal 62 (I). Signal 68 (-Q) is two-hundred-seventy degreesout of phase with signal 62 (I). Moreover, the amplitude of inputcurrent signals 62 (I) and 66 (-I) are substantially the same.

With respect to signals 90-96 associated with IF polyphase filter 14,signals 94 (-I) and 96 (-Q) are one-hundred-eighty degrees out of phasewith signals 90 (I) and 92 (Q). Output voltage signals 24 (I) and 28 (Q)form a first differential voltage signal at a first phase (e.g., 0degrees). Output voltage signals 26 (-I) and 30 (-Q) form a seconddifferential voltage signal at a second phase (e.g., 180 degrees) thatis phase shifted by one-hundred-eighty degrees from the first phase.

It should be understood that IF polyphase filter 14 of circuit 10 mayalso operate upon single-ended input signals to generate single-endedoutput signals.

FIG. 2 illustrates one embodiment of an image frequency rejectioncircuit 100 that includes a local oscillator 110, LO polyphase filter14, mixers 112 and 114, IF polyphase filter 14, and converter circuit120. In general, circuit 100 modifies the frequency of incoming RFsignals 122 using the frequency of LO signals 124 a and 124 b togenerate signals 126 a and 126 b at a desired intermediate frequency(IF). In doing so, circuit 100 uses the phase shifts associated with LOand IF polyphase filters 14 to cancel any image frequency signalsgenerated by mixers 112 and 114. A particular advantage of circuit 100is that either or both of LO polyphase filter 14 and IF polyphase filter14 operate in the current domain.

Local oscillator 110 comprises a device that generates signals 20 and 22at a selected frequency. Signals 20 and 22 form a differential voltagesignal, also referred to as an LO signal. Filter circuit 10 receivessignals 20 and 22, and generates a first LO signal 124 a and a second LOsignal 124 b that is ninety degrees out of phase with first LO signal124 a. The operation of filter circuit 10 is described in greater detailabove with reference to FIG. 1. Although LO signals 124 a and 124 b aredescribed as being generated by a current driven LO polyphase filter 14(as a part of filter circuit 10), it should be understood that LOpolyphase filter 14 may be voltage driven without departing from thescope of circuit 100.

Mixers 112 and 114 receive RF signals 122. In one embodiment, circuit100 forms a portion of a broadband integrated television tuner thatreceives RF signals 122 from a variety of sources, such as an antenna ora cable television line. In this embodiment, RF signals 122 may span thetelevision frequency band. Mixers 112 and 114 comprise bolometers,photoconductors, Schottky diodes, quantum non-linear devices (e.g. SISreceivers or Josephson junction mixers), variable gain amplifiers or anyother suitable device that multiplies RF signals with LO signals togenerate IF signals. For example, mixer 112 multiplies RF signals 122with LO signal 124 a to generate IF signals 130 and 132. Mixer 114multiplies RF signals 122 with LO signal 124 b to generate IF signals134 and 136. In doing so, mixers 112 and 114 may generate undesiredimage frequency signals (not shown) which can interfere with the desiredsignals. In a particular embodiment the image frequency signals comprise(RF+2*IF).

Signal 132 is substantially one hundred-eighty degrees out of phase withsignal 130. Signal 134 is ninety degrees out of phase with signal 130.Signal 136 is two hundred-seventy degrees out of phase with signal 130.In one embodiment, signals 130-136 propagate from mixers 112 and 114 toIF polyphase filter 14 in the current domain. IF polyphase filter 14receives 130-136 along with the unwanted image frequency signalsgenerated by mixers 112 and 114. The phase shifts associated with LOpolyphase filter 14 and IF polyphase filter 14 combine to removeunwanted frequencies, including the image frequencies, from the IFsignals. IF polyphase filter 14 generates output signals 140, 142, 144,and 146. Signals 140 and 144 combine to form signal 148. Signals 142 and146, which are one hundred-eighty degrees out of phase with signals 140and 144, combine to form signal 150. The detailed operation of IFpolyphase filter 14 is described above with reference to FIG. 1.

A particular advantage of one embodiment of circuit 100 is that IFpolyphase filter 14 operates upon current signals 130-136 to generatecurrent signals 140-146. By operating in the current domain, polyphasefilter 14 requires less power and circuit 100 removes current-to-voltageconversion circuitry and voltage buffers between mixers 112, 114 andpolyphase filter 14. This yields a significant savings in bias currentwhile still providing linearity within circuit 100. Moreover, parasiticcapacitances on intermediate lines within circuit 100 are no longersignificant design considerations.

Converter circuit 120 comprises any suitable circuitry that convertscurrent to voltage, current to power, or current to amplified current.For example, circuit 120 may comprise a transimpedance amplifier, acurrent amplifier, or any other suitable device to manipulate the formof signals 126 a and 126 b. Signals 126 a and 126 b comprise IF signalsrepresented as currents, voltages, or power based upon the type ofconverter circuit 120 that is used in circuit 100.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade hereto without departing from the sphere and scope of the inventionas defined by the appended claims.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims to invoke ¶ 6of 35 U.S.C. § 112 as it exists on the date of filing hereof unless“means for” or “step for” are used in the particular claim.

1. An image frequency rejection circuit, comprising: a first polyphasefilter operable to generate a first LO signal and a second LO signalthat is substantially ninety degrees out of phase with the first LOsignal; a first mixer operable to receive an RF signal and the first LOsignal, the first mixer further operable to communicate a first IFcurrent signal, a second IF current signal that is substantiallyone-hundred-eighty degrees out of phase with the first IF currentsignal, and at least a portion of an image frequency signal; a secondmixer operable to receive the RF signal and the second LO signal, thesecond mixer further operable to communicate a third IF current signalthat is substantially ninety degrees out of phase with the first IFcurrent signal, a fourth IF current signal that is substantiallytwo-hundred-seventy degrees out of phase with the first IF currentsignal, and at least a portion of the image frequency signal; and asecond polyphase filter communicatively coupled to the first mixer andthe second mixer, wherein a phase shift associated with the firstpolyphase filter and a phase shift associated with the second polyphasefilter combine to cancel the image frequency signal such that the secondpolyphase filter communicates a first IF output current signal, a secondIF output current signal that is substantially one-hundred-eightydegrees out of phase with the first IF output current signal, a third IFoutput current signal that is substantially in phase with the first IFoutput current signal, and a fourth IF output current signal that issubstantially one-hundred-eighty degrees out of phase with the first IFoutput current signal.
 2. The circuit of claim 1, further comprising acurrent-to-voltage converter circuit coupled to the second polyphasefilter and operable to generate a first IF voltage signal using thefirst and third IF current output signals, and to generate a second IFvoltage signal using the second and fourth IF current output signals,wherein the second IF voltage signal is substantially one-hundred-eightydegrees out of phase with the first IF voltage signal.
 3. The circuit ofclaim 1, further comprising: a first circuit component coupled to theinput of first polyphase filter and operable to propagate a first inputcurrent signal, a second input current signal, a third input currentsignal, and a fourth input current signal, wherein the third and fourthinput current signals are substantially one-hundred-eighty degrees outof phase with the first and second input current signals; wherein thefirst polyphase filter is further operable to: receive the first,second, third, and fourth input current signals; generate a first outputcurrent signal; generate a second output current signal that issubstantially ninety degrees out of phase with the first output currentsignal; generate a third output current signal that is substantiallyone-hundred-eighty degrees out of phase with the first output currentsignal; and generate a fourth output current signal that issubstantially two-hundred-seventy degrees out of phase with the firstoutput current signal; and wherein: the first output current signal andthe third output current signal form the first LO signal; and the secondoutput current signal and the fourth output current signal form thesecond LO signal.
 4. The circuit of claim 1, wherein the image frequencyrejection circuit forms a portion of a television tuner and the RFsignal spans at least a portion of a television frequency band.
 5. Animage frequency rejection circuit, comprising: a first polyphase filteroperable to: receive first, second, third, and fourth input currentsignals; generate a first output current signal; generate a secondoutput current signal that is substantially ninety degrees out of phasewith the first output current signal; generate a third output currentsignal that is substantially one-hundred-eighty degrees out of phasewith the first output current signal; and generate a fourth outputcurrent signal that is substantially two-hundred-seventy degrees out ofphase with the first output current signal; wherein the first outputcurrent signal and the third output current signal form a first LOsignal and the second output current signal and the fourth outputcurrent signal form a second LO signal that is substantially ninetydegrees out of phase with the first LO signal; a first mixer operable toreceive an RF signal and the first LO signal, the first mixer furtheroperable to communicate a first IF signal, a second IF signal that issubstantially one-hundred-eighty degrees out of phase with the first IFsignal, and at least a portion of an image frequency signal; a secondmixer operable to receive the RF signal and the second LO signal, thesecond mixer further operable to communicate a third IF signal that issubstantially ninety degrees out of phase with the first IF signal, afourth IF signal that is substantially two-hundred-seventy degrees outof phase with the first IF signal, and at least a portion of the imagefrequency signal; and a second polyphase filter communicatively coupledto the first mixer and the second mixer, wherein a phase shiftassociated with the first polyphase filter and a phase shift associatedwith the second polyphase filter combine to cancel the image frequencysignal such that the second polyphase filter communicates a first IFoutput signal, a second IF output signal that is substantiallyone-hundred-eighty degrees out of phase with the first IF output signal,a third IF output signal that is substantially in phase with the firstIF output signal, and a fourth IF output signal that is substantiallyone-hundred-eighty degrees out of phase with the first IF output signal.6. The circuit of claim 5, wherein the second polyphase filter iscurrent driven such that the first, second, third, and fourth IF signalscomprise current signals and the first, second, third, and fourth IFoutput signals comprise current signals.
 7. The circuit of claim 5,wherein the image frequency rejection circuit forms a portion of atelevision tuner and the RF signal spans at least a portion of atelevision frequency band.